Fiber- and fabric-reinforced resins have achieved broad usage because of their relatively high strength-to-weight ratio and have been incorporated in articles as diverse as automobiles and tennis racquets. Many polymers have been used as matrix resins, such as polycarbonates, epoxy resin, and polyesters generally.
It is highly desirable that such reinforced resins be able to be worked or molded in the field without loss of strength. One approach to this goal has been to use a low melting polymer wich can be cured or cross-linked to a thermostable resin upon being melted, thereby permitting working and shaping concurrent with attainment of maximum strength.
Where cross-linking involves a chemical reaction which liberates a volatile by-product, voids may be produced in the resulting polymer leading to reduction in strength and limitations on shaping. This disadvantage has been overcome using the biphenylene nucleus as a cross-linking entity. Difunctionalized biphenylene monomers have been incorporated into polyquinoxalines (Garapon and Stille, Macromolecules, 10, 627 (1977); Recca and Stille, ibid., 10, 1344 (1977) and 11,479 (1978)), polyamides (Swedo and Marvel, J. Polymer Sci., Polymer Chem. Ed., 16, 2711 (1978)) and polyaromatic ether-ketones and ether-ketone-sulfones (idem., ibid., 15, 683 (1977) and 17, 2815 (1979)). In U.S. Pat. No. 4,197,393 the patentees incorporated the biphenylene nucleus into the polymer chain itself and upon subsequent heating cross-linking occurred via the formed biphenylene diradical to afford a thermostable resin. U.S. Pat. No. 4,269,953 presents the variant wherein the biphenylene nucleus is incorporated as an additive which causes cross-linking of a thermoplastic polymer upon heating.
The solutions embodied in the cited art suffer from the disadvantage that the source of the biphenylene entity in each case is inordinately expensive. For example, the patentees of U.S. Pat. No. 4,197,393 require a biphenylene dicarboxylic acid halide, available only via a multi-step synthesis from biphenylene. The patentee of U.S. Pat. No. 4,269,953 requires additives of the structure ##STR1## whose availability also is limited by the necessity of its multi-step synthesis from the parent biphenylene. Recognizing the limitations accompanying these disadvantages I have recently described an alternate solution in U.S. Pat. No. 4,414,380 where the biphenylene moiety is introduced into the backbone of the polymer chain via acylation by a dicarboxylic acid where one of the acid groups of the latter acylates biphenylene and the other acylates a diphenyl either or di(phenoxyphenyl)sulfone in a step-growth polymer.
However, even the latter approach suffers from the drawback that a new copolymer must be synthesized to incorporate the biphenylene units. In this application I have taken an approach radically different from that in the prior art by appending monofunctional biphenylene monomers onto a preformed thermally stable polymer containing aromatic nuclei, generally via a Friedel-Crafts reaction. The result is a polymer quite similar to the original one but with the biphenylene moieties, which can effect cross-linking upon being cured at a temperature in excess of about 300.degree. C., pendant to the polymer chain or backbone.
The advantages of the current invention include the flexibility and convenience which result from starting with a commercially available polymer. Another advantage is that the current approach permits one to directly control cross-link density by the amount of biphenylene moieties incorporated, with the amount incorporated itself being readily controlled. Another advantage is that the instant approach has only a minimal effect on the properties of the polymer prior to cross-linking.